Blackout Cake

Make the chocolate filling

1. In a medium saucepan, heat the cocoa powder, hot water, sugar, and chocolate over low heat, stirring constantly until the cocoa powder dissolves and the chocolate melts. Add the dissolved cornstarch and the salt, increase the heat to medium-high, and boil gently for 1 minute, stirring constantly, especially where the sides and bottom of the pan meet.
2. Remove the pan from the heat, add the butter and vanilla, and stir until the butter melts. Pour the filling into a bowl and press plastic wrap onto the surface to prevent a skin from forming. Refrigerate until the filling thickens, at least 5 hours or up to overnight. It should be the consistency of soft pudding.

Make the chocolate cake

1. Position a rack in the middle of the oven. Preheat the oven to 350°F (180°C). Butter the bottom and sides of two 9-in round cake pans with 1 3/4- to 2-inch sides. Line the bottoms with parchment paper and butter the paper.
2. Place the chocolate, cocoa powder, and milk in a heatproof bowl (or the top of a double boiler) and place it over, but not touching, barely simmering water in a saucepan (or the bottom of the double boiler). Stir until the chocolate is melted, the cocoa powder dissolves, and the mixture is smooth. Remove from the heat and set aside to cool to lukewarm.
3. Meanwhile, sift the flour, baking powder, baking soda, and salt together in a medium bowl.
4. In a large bowl with an electric mixer on medium speed beat the butter and sugar until light in color and smooth, about 2 minutes. Stop the mixer and scrape the sides of the bowl as needed. Add 2 of the eggs and mix until each incorporated. Add the remaining 2 eggs and the vanilla and mix until incorporated. Beat for 1 minute more. Reduce the speed to low and mix in the cooled chocolate mixture until blended and smooth. Add the flour mixture, mixing just until the flour is incorporated and the batter looks smooth. Pour the batter into the prepared pans.
5. Bake just until the tops of the cakes feel firm when lightly touched and a toothpick inserted in the center of each cake comes out clean, about 40 minutes. Cool the cakes in the pans on wire racks for 10 minutes. Use a small, sharp knife to loosen each cake from the sides of the pan, and invert each cake onto a wire rack. Carefully remove the paper and place it loosely on the cake. Let the cake layers cool thoroughly, then discard the paper.

Make the chocolate frosting

1. Place the chocolate and butter in a heatproof bowl (or the top of a double boiler) and place it over, but not touching, barely simmering water in a saucepan (or the bottom of the double boiler). Stir until the chocolate and butter are melted and smooth. Remove from the heat and set aside to cool slightly. Check with your fingertips that the temperature of the lukewarm water feels similar to that of the chocolate. (It should be 88°F to 90°F (31°C to 32°C) when measured with an instant-read thermometer.)
2. Add the lukewarm water all at once to the chocolate mixture and gently whisk until the frosting is smooth. It will thicken slightly when the water is added. Stir in the corn syrup and vanilla. Let the frosting sit at room temperature until it cools and thickens slightly, about 30 minutes.

Frost the blackout cake

1. Invert a serving plate on top of one of the cake layers and turn it so the cake is top up. Tuck a strip of wax paper under the edge of the cake all the way around to keep the plate clean. Use a long serrated knife, cut the cake layer horizontally into two even layers, leaving the layers in place. Carefully slip the removable bottom of a tart pan or spring-form pan between the layers, lift the top layer, and set aside. Use a thin metal spatula to spread about half of the cold filling over the bottom half of the cake layer, leaving a 1/2-inch plain edge around. Carefully slide the top half of the cake layer on top of the filling, centering it over the bottom layer. Spread the remaining filling over the top.
2. Invert the second cake layer onto a plate so that it is top up, and cut it horizontally into two even layers. Carefully slide the top layer on top of the filling-topped stacked cake layers. You will have a three-layer cake. The remaining cake will be crumbled and used to cover the frosted cake.
3. Use the thin metal spatula to cover the top and sides of the cake with a thin layer of the frosting. Refrigerate the cake for 15 minutes to firm the cake and filling. Spread the remaining frosting over the top and sides of the chilled cake.
4. Using your fingers, crumble the remaining cake layer into small crumbs. Press the cake crumbs onto the side of the cake and, if desired, sprinkle them over the top of the cake. Remove the wax paper strips. If desired, sprinkle the top of the cake with chocolate shavings. Use a large, sharp knife to cut the cake. The cake can be covered and refrigerated for up to 2 days. Let it sit at room temperature for about 45 minutes before serving. Originally published April 12, 2010.